This invention pertains to photographic elements, and in particular to photographic elements incorporating blocked photographic developers in a novel arrangement of layers.
It is well known in the art that the introduction of conventional color developers, such as p-aminophenols, into sensitized photographic elements containing silver halide salts, leads to desensitization of the silver halide emulsion and unsuitable fog in such layers. Much effort has therefore been directed at trying to produce effective blocked developers, which can be introduced in silver halide emulsion elements without deleterious desensitization or fog effects and which unblock chemically under conditions of development so that developer is free to participate in color forming (dye forming) reactions.
U.S. Pat. No. 3,342,599, to Reeves, discloses the use of Schiff base developer precursors. Schleigh and Faul, in a Research Disclosure (129 (1975) pp. 27-30), described the quaternary blocking of color developer and the acetamido blocking of p-phenylenediamines. Subsequently, U.S. Pat. No. 4,157,915, to Hamaoka et al., and U.S. Pat. No. 4,060,418, to Waxman and Mourning, describe the preparation and use of carbamate blocked p-phenylenediamines.
All of these approaches and inventions have failed in practical product applications because of one or more of the following problems: desensitization of sensitized silver halide; unacceptably slow unblocking kinetics; instability of blocked developer yielding increased fog and/or decreased Dmax after storage.
Recent developments in blocking and switching chemistry have led to blocked p-phenylenediamines that perform well. In particular, compounds having ".beta.-ketoester" type blocking groups (strictly, .beta.-ketoacyl blocking groups) are described in U.S. Pat. No. 5,019,492. With the advent of the .beta.-ketoester blocking chemistry, it has become possible to incorporate p-phenylenediamine developers in film systems in a form from which they only become active when required for development.
The incorporation of these blocked developers in photographic elements is carried out using colloidal gelatin dispersions of the blocked developers. These dispersions are prepared using means well known in the art, wherein the developer precursor is dissolved in a high vapor pressure organic solvent (for example, ethyl acetate), along with, in some cases, a low vapor pressure organic solvent (such as dibutylphthalate), and then emulsified with an aqueous surfactant and gelatin solution. After emulsification, usually done with a colloid mill, the high vapor pressure organic solvent is removed by evaporation or by washing, as is well known in the art.
The .beta.-ketoacyl blocked developers are released from the film layers in which they are incorporated by an alkaline developing solution containing a dinucleophile, for example hydroxylamine.
There has been a need for a photographic element incorporating a blocked developer which is stable until development. Then the element can be developed rapidly and easily. There has also been a need for a process for developing an image in a photographic element which employs a developing solution having a simplified composition.